Field of the INvention
Magnetically responsive microparticles are useful in biological techniques requiring the separation of bound from free fractions. Magnetically responsive particles useful in immunoassays, for the separation of cells, as magnetic resonance imaging agents, etc. have been described in U.S. Pat. Nos. 4,770,183, 4,695,392, 4,329,241, 4,230,685, 4,177,253, 5,069,216, and 5,091,206, all incorporated herein by reference.
Preferably, magnetically responsive microparticles have negligible residual magnetism, and when used in immunoassays, preferably have diameters which cover the range of the visible light spectrum so as to facilitate heterogeneous and homogeneous immunoassay methods.
Unfortunately, prior art magnetically responsive microparticles have considerable residual magnetism due to the greater than single domain size of the magnetic materials utilized. Such residual magnetism causes clumping in the absence of a magnetic field causing the magnetic particles to fall out of Brownian motion and settle out of suspension quickly. Further, the large size of the prior art aggregates of magnetite used in magnetically responsive microparticles limits the overall size of the microparticle. For example, U.S. Pat. No. 5,091,206 provides 1 micron polymer cores coated with approximately 600-800 nm diameter particles of a magnetic metal oxide. Thus, the overall size of the particles is at least approximately 2.2 microns.